Continuous melt replenishment for crystal growth

ABSTRACT

The invention features a method of continuous crystalline growth. A granular source material is introduced into a hopper. A volume of the granular source material exiting the hopper is disposed on a translationally moving belt. The volume of the granular source material forms an angle of repose with the moving belt. The granular source material disposed on the moving belt is continuously fed into a crucible comprising a melt of the granular source material at a rate based on the angle of repose, the speed of the belt, and the size of the opening of the hopper. A crystalline ribbon is continuously grown by solidifying the melt.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.09/304,284 filed on May 3, 1999 now U.S. Pat. No. 6,090,199.

FIELD OF THE INVENTION

The invention relates generally to crystal growth of semiconductormaterials, and more specifically to a method of continuous crystalgrowth.

BACKGROUND

In order to produce lower cost solar cells to facilitate large scaleelectrical applications of solar electricity, it is important to providelower cost substrate materials for making the solar cells. A knownmethod for achieving this objective is to grow crystalline silicon usinga continuous ribbon growth process as described in U.S. Pat. Nos.4,661,200; 4,627,887; 4,689,109; and 4,594,229.

According to the continuous ribbon growth method, two high temperaturematerial strings are introduced through holes in a crucible whichcontains a shallow layer of molten silicon. A crystalline silicon ribbonforms as the melt solidifies while being pulled vertically from themelt. The strings stabilize the edges of the growing ribbon. The moltensilicon freezes into a solid ribbon just above the layer of moltensilicon. To make this ribbon silicon process continuous, silicon isadded to the melt as the crystalline silicon is formed to keep theamount of melt constant. Keeping the amount of the melt constant duringthe growth process is also important in order to achieve uniform andcontrollable growth of the crystalline silicon, and to keep the thermalenvironment of the cooling ribbon constant. Slight changes in the depthof the melt and consequent changes in the vertical position of thesolid-liquid interface can significantly change this thermalenvironment. For example, it has been found that variations in the meltdepth of more than about one millimeter can result in a markedlydifferent thickness and introduce a residual stress state of the grownsilicon ribbon. For all of these reasons, a constant melt level is animportant element in insuring uniform, controlled growth of siliconribbon. A method for continuously measuring the melt depth to providefeedback to a feed mechanism can be accomplished as described in theco-pending patent application titled, “Melt Depth Control forSemiconductor Materials Grown from a Melt,” which is incorporated hereinby reference. Once such a method is established, it then becomesimportant to be able to introduce a feed material at an accurate andpredetermined rate. Since the solid silicon feed material must bemelted, it is important that the introduction of solid silicon into themelt takes place with a minimum of thermal disruption in the immediateenvironment of the solid-liquid interface.

Controllable metering of feed material also has general application tocrystal growth, where thermal upsets are not desirable, and a constantfeed rate is needed. One example is in Czochralski growth of Si ingots,where it is desired to introduce additional feed material to thecrucible during the growth. In this manner, a longer ingot of Si can bepulled from a single seeding.

Several methods of introducing a feed material into a cruciblecontaining molten silicon are known U.S. Pat. No. 4,036,595 describes amethod wherein a separately heated crucible is used. European Patent No.0 170 856 B1 describes a feeder with a moving belt which can be used foradding the feed silicon to a rotating cylindrical crucible withconcentric dams to collect and melt the feed silicon in Czochralskicrystal growth. U.S. Pat. No. 5,242,667 teaches a method which uses asilicon wiper blade on a horizontal, silicon rotating disc to controlthe feed rate from a storage hopper above the disc. All of thesemethods, however, have disadvantages. The first method requires aseparately heated crucible. The second method requires a complexcrucible arrangement that is only suitable for a cylindrical geometry,and may be difficult to implement for a feed material with a low angleof repose. The third method has limitations in controlling the feed ratefor small feed rates such as would be found in silicon ribbon growth.

In order to controllably and continuously transport a silicon feedmaterial for continuous silicon ribbon growth, it is important that thematerial have a morphology which lends itself to being readily andcontrollably transported. Silicon itself, if crushed, exhibits angularfracture and breakage along cleavage planes. This renders crushedsilicon to be highly irregular in shape and thus difficult tocontrollably transport using a known method such as a vibratory feeder.Spherical silicon, on the other hand, can be produced by the fluidizedbed decomposition of silane (SiH₄) or in a shot tower. The former is awidely used method and presently is the source material for most siliconribbon growth. This method produces spherical silicon with a sizedistribution from almost fine dust to about 2 mm in diameter.

U.S. Pat. No. 5,098,229 describes a silicon melt replenishment systemwhich uses a pressurized fluid to blow silicon spheres up into a melt.One major disadvantage of this system is that it requires sphericalsilicon. Another major disadvantage is that the silicon spheres need tobe in a fairly narrow particle size range to be effective. Sphericalparticles either too small or too large cannot be effectively used withthis system. The result is that it is necessary to sieve the fluidizedbed material to eliminate too large and too fine silicon spheres. Theadditional labor and handling of the sieving operation add to cost andincrease the risk of impurity contamination.

Silicon ribbon grown for solar cell purposes is usually doped with asmall amount of a dopant, typically Boron. Very small silicon pelletscan be doped with Boron. Alternatively, very small amounts of Boronpellets can be added to the feed material consisting of silicon pelletsand physically mixed with the silicon pellets before being transportedinto the molten silicon prior to growth. The mixing is done to promote auniform distribution of Boron. However, in some transport systems suchas a vibratory feeder, inhomogeneous mixing of Boron pellets or Boroncontaining silicon pellets can result. This can produce variations inthe final bulk resistivity of the grown ribbon and this in turn canresult in a less tightly controlled manufacturing process for solarcells.

A melt replenishment method that allows for continuous growth, whichutilizes all sizes and morphologies of silicon, which allows for asimplified, low cost crucible design for silicon ribbon growth, andwhich produces a more homogeneous mixing of Boron, is thus very muchneeded and would represent a significant step towards low costphotovoltaics.

SUMMARY OF THE INVENTION

The invention features a method of continuous crystalline growth. In oneaspect, the invention features a method of continuous crystalline ribbongrowth. A granular source material is introduced into a feeder. In oneembodiment, the feeder is a hopper. A volume of the granular sourcematerial exiting the hopper is disposed on a translationally movingbelt. The volume of the granular source material forms an angle ofrepose with the moving belt. The granular source material disposed onthe moving belt is continuously fed into a crucible comprising a melt ofthe granular source material at a rate based on the angle of repose, thebelt speed, and the hopper opening size. A crystalline ribbon iscontinuously grown by solidifying the melt at the solid liquidinterface.

In one embodiment, a semiconductor is the granular source material and acrystalline semiconductor is grown in a continuous ribbon. Thesemiconductor source material can be doped n or p type and is fed fromthe hopper onto the moving belt which then feeds into the crucible.

In another preferred embodiment, silicon is the granular sourcematerial. The silicon source material can be doped either n or p type.The silicon granular source material is introduced into a hopper and thematerial exits the hopper onto a moving belt. The belt speed varies fromabout 2 mm/min to 10 mm/min. The material exits the moving belt and isfed into a growth crucible. The rate of feeding the source material intothe growth crucible can be based on the angle of repose, the speed ofthe moving belt, and the hopper opening size just above the belt.

In another aspect, the invention features a method of introducing asource material into a melt for use in a continuous crystalline growth.A granular source material is introduced into a hopper. A volume of thegranular source material exiting the hopper is disposed on atranslationally moving belt. The belt has at least one raised lip. Inone detailed embodiment, the raised lip is disposed near an edge of thebelt. The volume of the granular source material forms an angle ofrepose with the moving belt. The granular source material disposed onthe moving belt is continuously fed in a crucible comprising a melt ofthe granular source material at a rate based on the angle of repose.

The invention also features a system for continuous ribbon crystalgrowth. The system includes a hopper for providing a granular sourcematerial in a pile forming an angle of repose with a substantiallyplanar surface, a crucible for holding the melt of the granular sourcematerial, a pair of strings disposed through the crucible forstabilizing crystalline ribbon growth from the melt and atranslationally moving belt. The belt continuously delivers the granularsource material from the feeder to the crucible at a rate based on theangle of repose, the belt speed, and the hopper opening just above thebelt. In one embodiment, the belt comprises at least one raised lip. Inanother embodiment, the belt comprises a pair of lips, and furthercomprises a stopper disposed above the belt, behind the hopper.

In another embodiment, the feeder comprises a hopper and the granularsource material comprises a semiconductor material, wherein the granularsource material can also have non-uniform sized and shaped particles.

In another aspect, the invention features a system for introducing asource material into a melt for use in a crystalline growth system. Thesystem comprises a feeder for providing a granular source material in apile formning an angle of repose with a substantially planar surface, acrucible for holding a melt of the granular source material, and atranslationally moving belt for continuously delivering the granularsource material from the feeder to the crucible at a rate based on theangle of repose. The belt comprises at least one raised lip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an angle of repose of a granular material.

FIGS. 2(a)-2(c) illustrate variations in the angle of repose forspherical particles with a narrow size distribution, spherical particleswith a wide size distribution, and crushed silicon particles having anangular shape.

FIG. 3 shows an embodiment of a portion of a continuous crystal growthsystem according to the present invention.

FIG. 4 illustrates a system for continuous ribbon crystal growth.

FIG. 5 shows an embodiment of a continuous ribbon crystal growth systemaccording to the present invention.

FIG. 6 illustrates calculation of the cross sectional mass area of apile of granular material on a horizontal plane.

FIG. 7 is a graph illustrating the linear relationship between the feedrate and the belt speed (or % power) for the continuous ribbon crystalgrowth system of the invention, and the highly non-linear relationshipbetween the feed rate and the belt speed (or % power) for a vibratoryfeeding system.

FIG. 8 shows the degree of variation which a vibratory feeder canintroduce when compared to the present invention.

FIGS. 9a and 9 b illustrate one embodiment of part of a continuouscrystal growth system according to the present invention.

FIGS. 10a and 10 b show cross sectional views of part of the crystalgrowth system of FIGS. 9a and 9 b, respectively.

FIG. 11 illustrates the calculation of the cross sectional mass area ofa pile of granular material on a belt of the crystal growth system ofFIGS. 9a and 9 b.

DETAILED DESCRIPTION

All granular materials have a characteristic angle, termed the angle ofrepose, which refers to the angle with a horizontal plane on which afree standing pile of the granular material would form if allowed toslowly fall from a narrow orifice onto the plane. Such an angle is abasic property of a material having a given particle size distributionand a given particle shape distribution.

Referring to FIG. 1, a pile of granular material 11 exiting a hopper 12or a large funnel shaped feeder forms a specific angle of repose 13 witha flat surface 15. For a particular particle size and morphologydistribution, a constant cross section or a constant mass of material 14is repeated as illustrated in FIG. 1, and shown in detail further inFIG. 6.

Referring to FIGS. 2(a) to 2(c), the angle of repose 23 a, 23 b and 23 cvaries for different particle size and morphology distribution. FIG.2(a) shows the angle of repose for particles that are spherical andhaving a narrow size distribution. FIG. 2(b) shows the angle of reposefor particles that are spherical and having a wide size distribution.The angle of repose for the particles with a wide size distribution isdifferent than the angle of repose for the particles with a small sizedistribution. FIG. 2(c) shows the angle of repose for non-sphericalshaped particles. The particles are angular shaped. The angle of reposeof non-spherical particles is larger than the angle of repose ofspherical particles.

The present invention uses the concept of the angle of repose formed ona slowly moving belt to provide continuous, uniform melt replenishmentfor the growth of silicon ribbon. A granular source material forming ona translationally moving belt forms an angle of repose with the movingbelt and a constant cross section as described above. The rate ofintroducing the source material into a crucible containing a melt can becontrolled by adjusting the belt speed, the angle of repose, and thesize of the hopper opening just above the belt.

Referring to FIG. 4, a continuous ribbon growth system 50 includes acrucible 43 containing a pool of molten silicon (“the melt”) 42 and apair of strings 44 extending through the crucible 43. A thinpolycrystalline sheet of silicon 41 is slowly drawn from the melt 42, asthe cooler liquid silicon crystallizes at the top of meniscus. Thestrings 44 passing through holes (not shown) in the bottom of thecrucible 43 become incorporated in and define the edge boundaries of thecrystalline sheet 41. The strings 44 stabilize the edges as the sheet 41grows. The surface tension of the silicon prevents leaks through theholes of the crucible 43 where the strings 44 pass through. In an actualcontinuous-crystal-growth apparatus, the melt 42 and the crucible 43 arehoused within an inert-gas filled housing (not shown) to preventoxidation of the molten silicon. Rollers (not shown) keep the sheet 41moving vertically as the sheet 41 grows. The crucible 43 remains heatedto keep the silicon molten in the melt 42. The crucible 43 also remainsstationary.

To provide a continuous crystal growth process, the melt 42 needs to becontinuously replenished as some of the melt 42 is lost by solidifyingto form a crystalline ribbon. In one embodiment, a granular sourcematerial 35 enters a hopper 32 through a large opening 10 and exits thehopper 32 through a small opening 8, as shown in FIG. 3. The granularsource material 35 can have non-uniform sized particles. The sourcematerial, for example, can be a semiconductor material. In oneembodiment, the source material comprises silicon and a dopant such asboron. The source material 35 exiting the hopper 32 disposes a pile ofthe source material 35 on a translationally moving belt 34. The pile ofthe source material 35 has an angle of repose, which is determined bythe shape and size distributions of the particles forming the sourcematerial 35.

Referring to FIG. 5, the source material 35 disposed on the moving belt34 is continuously fed into a crucible 43 comprising a melt of thesource material 35 at a predetermined rate. The source material isintroduced into the crucible through a funnel 45 and a tube 47. The tube47 is positioned inside one end of the crucible 35. In one embodiment,the feed rate is constant. The feed rate is based on the angle of reposeof the source material.

The volume of the source material is equal to the product of the masscross sectional area of the source material times the speed of themoving belt. FIG. 6 illustrates the mass cross sectional area. The masscross sectional area is equal to H²/tanα+HL, where α is the angle ofrepose, H is the height, and L is the size of the hopper opening justabove the belt. The feed rate can be controlled by the rate of movementof the belt in combination with the angle of repose. The belt, forexample, can move at a constant rate and thereby introduce the sourcematerial into the crucible at a constant rate. The belt can also move ata rate in the range from 2 mm/min to 10 mm/min. In still anotherembodiment, the feed rate is based on the cross sectional area of thesource material as it resides on the belt, or on the belt speed.

The use of the angle of repose concept along with a slowly moving belt,as shown in FIGS. 3, 5 and 6, enables one to transport continuously andvery accurately, any desired amount of granular silicon into a cruciblecontaining the melt. As is evident from FIG. 1, the cross-section ofmaterial emerging from the narrow orifice is constant due to theconstancy of the characteristic angle of repose. With this constantcross-section, the amount of material transported can be varied simplyby varying the speed of the belt carrying the granular material. Thus,the invention allows a considerable amount of latitude in choosing theparticle size distribution and the particle shape distribution for thesilicon feeder material.

A very simple, linear relationship exists between the belt speed and thefeed rate. FIG. 7 illustrates that the rate of introducing the sourcematerial 35 into the crucible 43 is directly proportional to the speedat which the belt 34 moves or the amount of power applied to the belt 34to move the belt 34. As shown in FIG. 8, the relationship between theapplied power or belt speed and the feed rate in the present inventionis also consistent, so that the feed rate resulting from a particularbelt speed can be repeated. In contrast, the relationship between thepower applied and the feed rate in a vibratory system is non-linear andcomplex, in addition to being inconsistent. The same amount of powerapplied to the vibratory system can result in varying feed rates in avibratory system as shown in FIG. 8.

In one embodiment, the location of the hopper exit is no less than 1inch from the (feeding) end of the belt. That is, the belt with thefeedstock deposited on it should travel at least 1 inch before thefeedstock drops off. This is to allow the pile of feedstock to extend toits limits (as governed by angle-of-repose) in the direction of belttravel, while the belt is not moving. Without this buffer distance,there is the risk of feedstock spilling over the end of the belt evenwithout belt motion—compromising the entire metering scheme. Thisconcept is illustrated in FIGS. 9a-9 b, where FIG. 9a shows a feedstockmaterial 108′ having an angle of repose twice that of the feedstockmaterial 108″ shown in FIG. 9b.

In another embodiment, small pulleys are used to move the belt. Thisembodiment ensures a well-defined point at which the feedstock slidesoff the belt. In one embodiment, the belt makes close to a 90° bend todefine the line where the feedstock drops into the funnel.

In another embodiment, the spacing between the belt and the hopper exitis about the maximum diameter of the feedstock particles. This serves tolimit the amount of feedstock on the belt, and allows the largestparticles to pass between the belt and the hopper exit.

In still another embodiment, the belt is flexible. The flexible beltallows for simple tensioning to define the resting belt position, whileallowing belt compliance if a feedstock particle with a large diameteris encountered. This compliance of the belt is useful for feedstock witha poorly defined particle size.

In still another embodiment, the belt is made of a material that doesnot contaminate the silicon feedstock with transition metals such asTitanium or Vanadium. The belt material can also be abrasion resistant,so that the belt material does not contaminate the melt.

Referring to FIGS. 9a-9 b and 10 a-10 b, a continuous crystal growthsystem 100 includes a belt 102 having raised lips 104 at the sides and astopper 106. The stopper 106 is positioned behind the hopper 108 in thedirection away from the belt motion 107. This embodiment is particularlyuseful when feeding a material having a low angle of repose or a poorlydefined angle of repose. The raised lips 104 and the stopper 106constrain the extent of the pile of feedstock 108′, 108″ that formsbelow the hopper 110. The raised lips 104 and the stopper 106 provideseveral advantages. They limit the amount of the feedstock 108′, 108″material present on the belt 102, providing controlled feeding of asmall mass flow of the feedstock 108′, 108″ material into the crucible(not shown). They also limit the size of the belt apparatus, and confinethe flow of the feedstock 108′, 108″ on the belt 102 in only thedirection of belt motion 107. The feedstock 108′, 108″ material isprevented from falling off the sides of the belt 102 or behind the belt102. FIG. 10a shows a feedstock 108′ having an angle of repose of α′placed on a moving belt 102 having raised lips 104 and a stopper 106.Alternatively, other embodiments can be used to restrict the size of thepile of the feedstock material. For example, a groove or a channel canbe created on the belt.

FIG. 10b shows a feedstock 108″ having an angle of repose of α″, whichis about half the angle of repose of α′, placed on a moving belt 102having raised lips 104 and a stopper 106. The lateral extent of the pileof feedstock 108″ having the lower angle of repose α″ would be quitelarge, if not constrained by the raised lips 104.

The angle of repose concept extends to the use of a belt with the raisedlips at the sides, as well as a belt with a channel or groove formed onits top surface. These features provide a positive limit to the lateralextent of the pile of feedstock at the exit of the hopper. This servesto modify the cross section of the pile to that shown in FIG. 6. Thearea of this pile is described by:

(L₂H₂)−{(tan α)(L₁L₂)²}

This calculation is illustrated in FIG. 11. The advantage here is thatthe overall width (L₂) of the pile can now be arbitrarily specified,where with the flat belt, this width depends on the angle of repose ofthe feedstock material.

In the present invention, the rate of introducing the source materialcan be controlled by adjusting the belt speed and is based on the angleof repose of the source material. This feature allows controllable andcontinuous replenishment of the melt, which in turn allows uniform,continuous crystal growth. Although the present invention has beenillustrated with reference to a continuous ribbon crystal growth methodand system, the invention is applicable to any crystalline growth systemin which controlled replenishment of the feedstock material isdesirable. For example, the present invention can be used withCzochralski crystal growth, Edge-Defined Film Growth (EFG dendritic webgrowth (WEB), or with solvent-metal assisted liquid phase growthsystems.

Equivalents

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of continuous crystalline ribbon growthcomprising the steps of: a) introducing a granular source material intoa feeder; b) disposing a volume of the granular source material exitingthe feeder on a translationally moving belt, the volume of the granularsource material forming an angle of repose with the moving belt; c)continuously feeding the granular source material disposed on the movingbelt into a crucible comprising a melt of the granular source materialat a rate based on the angle of repose; and d) continuously growing acrystalline ribbon by solidifying the melt at a solid liquid interface.2. The method of claim 1 wherein step a) comprises introducing a dopedsemiconductor source material into the feeder.
 3. The method of claim 2wherein the semiconductor source material comprises doped silicon. 4.The method of claim 1 wherein step a) comprises introducing a granularsource material into a hopper.
 5. The method of claim 4 wherein step c)comprises feeding the granular source material into the crucible at arate based on the angle of repose, a rate of movement of the movingbelt, and a size of an opening of the hopper.
 6. The method of claim 5wherein the rate of feeding the granular source material into thecrucible and the rate of movement of the moving belt have a linearrelationship.
 7. The method of claim 1 wherein step b) comprisesdisposing a volume of the granular source material from a belt at a rateof from about 0.1 g/min to about 100 g/min.
 8. The method of claim 1wherein step b) comprises disposing a volume of the granular sourcematerial on a moving belt which is moving at a substantially constantrate.
 9. The method of claim 1 wherein step c) comprises feeding thegranular source material into the crucible at a constant rate.
 10. Themethod of claim 1 further comprising the step of maintaining the depthof the melt at a constant level.
 11. The method of claim 1 wherein stepd) comprises forming the crystalline ribbon by solidifying the meltbetween a pair of strings passing through the crucible.
 12. The methodof claim 1 wherein step b) comprises disposing a volume of the granularsource material on a translationally moving belt having at least oneraised lip.
 13. The method of claim 1 wherein step b) comprisesdisposing a volume of the granular source material on a translationallymoving belt having a stopper positioned above the belt behind thefeeder.
 14. The method of claim 12 wherein step c) comprisescontinuously feeding the granular source material based on the angle ofrepose and a distance between a pair of lips.
 15. A method ofintroducing a source material into a melt for use in a continuouscrystalline growth comprising the steps of: a) introducing a granularsource material into a hopper; b) disposing a volume of the granularsource material exiting the hopper on a translationally moving belthaving at least one raised lip disposed near an edge of the belt, thevolume of the granular source material forming an angle of repose withthe moving belt; c) continuously feeding the granular source materialdisposed on the moving belt in a crucible comprising a melt of thegranular source material at a rate based on the angle of repose.
 16. Themethod of claim 15 wherein step b) comprises disposing the volume of thegranular source material on a translationally moving belt comprising apair of raised lips, and step (c) comprises continuously feeding thegranular source material at a rate based on the angle of repose and adistance between the pair of raised lips.
 17. The method of claim 15wherein step (c) comprises continuously feeding the granular sourcematerial at a rate based on a cross sectional area of the granularsource material disposed on the moving belt.
 18. The method of claim 15wherein step (b) comprises disposing a volume of the granular sourcematerial on the moving belt comprising a stopper disposed above the beltbehind the hopper.